Dental composites are being used increasingly for filling teeth, in order to circumvent possible side-effects of amalgam fillings, and in order to achieve a better aesthetic appearance. Dental composites consist as a rule of an inorganic material and an organic synthetic resin binder. The inorganic content consists mostly of glass powder. The glass powder must satisfy certain requirements as to the physical and chemical properties of the glass to be used for the powder in addition to the qualities necessary for a good filling.
The glass powder must first have a high strength. Also, the index of refraction of the filling must be matched to that of the synthetic resin in order to assure that the tooth filling will satisfy the aesthetic requirements, i.e., that it will be scarcely distinct from the dental enamel. It is furthermore important that the thermal expansion of the glass in the range in which the filling is used, i.e., at temperatures between 30.degree. C. and 70.degree. C., will match that of the substance of the tooth, so as to assure that the filling will have a sufficient resistance to temperature changes. The danger is precisely that the alternation between cold and hot foods might destroy its integrity. A minimal coefficient of expansion is commonly sought, because the comparatively high thermal expansion of the synthetic resin binder can thereby be compensated.
Furthermore, the tooth filling must be clearly distinct in its x-ray image from that of the tooth material. This signifies that the glass must have a certain minimum of x-ray opacity. According to ISO 4049: 1988 (E) the x-ray opacity of a filling of a thickness of 2 mm must be greater than that of an aluminum plate of equal thickness. The x-ray opacity of the filling is expressed as the aluminum equivalent thickness. The aluminum equivalent thickness is understood as the thickness of an aluminum plate which has the same x-ray absorption as a plate of filling material 2 mm thick. An aluminum equivalent thickness of 3 accordingly means that a 2 mm thick filling produces the same absorption as an aluminum plate 3 mm thick.
Also, the glass powder should be chemically stable against water, acids and alkalies to contribute to a long life of the filling. Due to possible toxic side effects, the use of barium components in the glass is avoided, although these components produce good x-ray opacity. The use of components containing lead is fundamentally forbidden for reasons of toxicity.
A fluoroaluminum silicate glass powder for use in dentistry is disclosed in U.S. Pat. No. 4,775,592. The basic glasses for this powder however have a very poor crystallization stability, so that their production is very difficult. The glasses have a very high fluorine content of about 10 to 40 wt-% and to achieve sufficient hardness and chemical stability they must contain Al.sub.2 O.sub.3 in amounts of up to 40% by weight. Even then, the hydrolytic stability of the glass powder is not satisfactory.
Furthermore, a barium-free dental glass with a high x-ray absorption is disclosed in the older German patent application P 43 23 143.7, and has a composition in percentage by weight of oxides of SiO.sub.2 45-65%; B.sub.2 O.sub.3 5-20%; Al.sub.2 O.sub.3 5-20%; CaO 0-10%; SrO 15-35% and F.sub.2 O 0-2%. The good x-ray opacity is achieved here by a comparatively high content of SrO.
An object of the invention is to provide a dental glass having a good x-ray absorption, which is free of barium and lead, and which has a good chemical and thermal stability.